1. Field of the Invention
The present invention relates to a construction machine having a multi-articulated front device, and more particularly to a front control system for a construction machine, e.g., a hydraulic excavator having a front device comprising a plurality of front members such as an arm, a boom and a bucket, which system modifies a signal from at least one control lever unit and controls the operation of the front device for performing area limiting excavation control to limit an area where the front device is allowed to move, locus limiting excavation control to move an end of the front device along a predetermined locus, etc.
2. Description of the Related Art
There is known a hydraulic excavator as typical one of construction machines. In a hydraulic excavator, front members, such as a boom and an arm, making up a front device are operated by an operator manipulating respective manual control levers. However, because the front members are coupled to each other through articulations for relative rotation, it is very difficult to carry out excavation work within a predetermined area or in a predetermined plane by operating the front members. Also, there is known a hydraulic excavator with a front device including an offset (second boom) for providing a wider excavation area, or a very small swivel-type hydraulic excavator capable of swiveling within a body width. But such a hydraulic excavator accompanies a risk that a front device may interfere with a cab depending on its posture.
In view of the above-mentioned state of art, various proposals for facilitating excavation work or preventing interference between the front device and the cab have been made.
For example, JP-A-4-136324 proposes that, with the aid of a slowdown area set in a position before reaching an entrance forbidden area, a front device is slowed down by reducing an operation signal input from a control lever when a part, e.g., a bucket, of the front device enters the slowdown area, and is stopped when the bucket reaches the boundary of the entrance forbidden area.
Also, WO95/30059 proposes that an excavation area is set beforehand, and a part, e.g., a bucket, of a front device is controlled to slow down its movement only in the direction toward the excavation area when the bucket comes close to the boundary of the excavation area, and to be able to move along the boundary of the excavation area without going out of the excavation area when the bucket reaches the boundary of the excavation area. More specifically, to realize the above control, the position and posture of each of front members, such as a boom and an arm, are calculated based on signals from position detecting means, e.g., angle sensors. Operating speeds (e.g., speeds of a boom cylinder, an arm cylinder, etc.) at which the front members, such as a boom and an arm, are moved in accordance with signals from respective control lever units are estimated based on calculated values of the position and posture of each of the front members, as well as the signals from the respective control lever units. Then, the signals from the respective control lever units are modified in consideration of the estimated operating speeds.
Further, WO95/33100 proposes that, in the area limiting excavation control system disclosed in the above-cited WO95/30059, respective load pressures of hydraulic actuators such as a boom cylinder and an arm cylinder are detected, and the signals from the respective control lever units are modified in consideration of the detected load pressures as well, thus enabling the bucket to be controlled with high accuracy regardless of change in the load pressures of the hydraulic actuators.